1. Field of the Invention
This invention relates to a subject positioning device for an optical interferometer and, more particularly, to a subject positioning device for positioning a test surface of a thine plate of subject within the axial extent of interference positions for interference in an object beam path of an optical interferometer whose object and reference beams have a short coherence length.
2. Description of Related Art
In interferometers such as Michelson interferometers, a reference beam and a subject beam into which a parallel beam of coherent light is divided by means of, for instance, a beam splitter are directed to and reflected by a reference surface and a test surface of a thin plate of subject, respectively. These reflected reference and subject beams are, thereafter, united by the beam splitter to interfere with each other, forming a pattern of interference fringes on an observation screen for examination of surface geometry or irregularity of the test surface of the subject.
In cases where using a laser beam to form interference fringes based on which surface geometry or irregularity examination of a test subject is made, because of a long coherence length of the laser light, it is unnecessary to position the test surface precisely with respect to the reference surface. There is a problem, if a thin transparent plate such as a glass plate, that the subject beam reflected by the surface opposite to the test surface of the subject interferes with both the reference beam and the subject beam reflected by the test surface of the subject to form interference fringes as noise in addition to essential interference fringes. In order for such a laser interferometer to avoid a ghost caused by the opposite surface of the subject which is superposed on the essential interference fringes necessary to a surface irregularity examination of, for example, a thin glass plate, the opposite surface of the thin glass plate is necessarily covered with a coat of, for instance, a refractive index matching oil, which is always undesirable from a viewpoint of handling a test subject and/or contamination of a test surface of the subject. Further, if a transparent test subject is considerably thin, a coat of a refractive index matching oil or the like exerts surface tension over the opposite surface of the test subject with an adverse effect of surface distortion, resulting in inaccurate surface examination.
While it is desirable for surface irregularity examination of a subject, such as thin glass plates and the like, to use coherent light having a short coherence length shorter than the double of the thickness of the subject and to position the test surface of the subject within the axial extent of interference positions less than the coherent distance. For example, in cases where using a red light emitting diode (red LED), which has a coherence length of approximately 30 .mu.m, as a light source, the axial extent of interference positions is less than only 15 .mu.m. Unless the test surface is positioned within the small axial extent of interference positions, the pattern of interference fringes which demonstrates surface geometry or irregularities from the reference surface existing on the test surface cannot be formed.